I didn't say a AA battery is the only option. There could be players powered by a AAA battery, or players powered by an easily swappable lion battery, such as one that is used by a cell phone or digital camera. While I would prefer the use of a AA or AAA battery, an easily swappable, easy to find, inexpensive lithium ion battery that is charged outside the player would also be good. While having a battery that is removable might add a bit to the player's size, if it is charged outside the player then the charging circuitry could be omitted, saving space.

I don't know of any current model Canon digital cameras that use a built in battery. They either use AA batteries, or easily swappable lithium ion batteries. Most cell phones use an easily swappable battery.

A Sansa Clip+ takes around 3 hours for a full charge and plays for around 12 hours. Play time is around 4 times charge time. With 2900 mah AA batteries, a player might be designed to play for 40 hours on one of these. If a 15 minute charger is used to charge 4 at a time, then play time would be 640 times charge time!

Some people use their mp3 player to play lectures. When using a player for more serious purposes than just listening to music, running out of power is a more serious concern. I guess your usage pattern of your mp3 player has not made this a factor for you, but for others running out of power is a serious concern. Having a player with a swappable battery and carrying a spare battery or two solves this problem. The alternative if one is forced to use players with a built in battery is to have players with a card slot, then have the files on a card, and carry a spare player. Carrying a spare battery is smaller and less expensive than carrying a spare player.

I think a player designed with a 3.7 volt input, that comes with a battery case for a 18650 battery would be cool. The 18650 battery is 3.7 volts, around 2600 mah and only around $5 each retail. A charger that charges one or two at a time is around $10 retail. The 18650 battery is just slightly larger than a AA battery. It could power a Clip+ for around 95 hours!

It's been a while since we first saw the $199 NorhTech Gecko hit the scene, but the AA-powered netbook looks to finally be hitting US shores in proper fashion, and the crew at Lilliputing just got their tester. Inside it's no great shakes, with a 1GHz Xcore86 processor, 512MB of RAM, an 8GB SD card, and an 8.9-inch 1024 x 600 display -- but almost every component can be swapped out for a more powerful variant, including the processor. That's certainly intriguing, especially for the education market NorhTech is targeting, but we're mostly on the edge of our seats waiting to find out how long those AAs last under some real use. Check one video after the break and another at the read link.

of all the threads you could have bumped....
Sadly, I actually like that little netbook. If only because I was shown the light on how irritating they can be, so its all about USPs now, and that one seems to have quite a few